Massive forest fires in 2000, which encircled the famed nuclear lab where the first atomic bomb was made, were mostly to blame because the blazes destroyed large amounts of vegetation in the area, said Goldstein.

He added added that without vegetation to hold the sediment, the runoff "flows directly into the Rio Grande."

Lab officials said the plutonium levels pose no threat to human health or to the environment, and were to be expected following the devastating Cerro Grande Fire in the summer of 2000.

"Much of the plutonium we're seeing is the result of former lab operations conducted during the Cold War," said lab spokesman James Rickman, noting that two separate studies of the possible runoff risks began immediately following the Cerro Grande fire.

"The bottom line is that even in the worst-case scenario, we found there is not an elevated risk to human health or the environment from exposure to these radionuclides," he said.

Plutonium-239 is a radioactive, man-made element produced since the 1940s for use in nuclear weapons and it has a radioactive half-life of 24,000 years.